The invention relates to a method for an override closed loop control of several controlled variables, whereby this override control works on the delay angle of a current converter, and to a device to this end.
To maintain a specific current in the operation of a current converter, it is sometimes necessary to have a current control with controller parameters (for example, the reset time TN and the proportional component KP of a PI (proportional-plus integral controller), which are adapted to the system. Often, secondary conditions must also be adhered to, for example if a specified extinction angle must not be undershot in the inverter operation, to avoid conduction-through, and/or when certain limiting values must be adhered to for the output voltage. As a manipulated variable, the current converter itself possesses virtually only the delay angle from which the firing pulses of its current-converter valves are derived. Therefore, to adhere to the secondary conditions, the transition must possibly be made from the current control to a corresponding closed-loop control of another internal state variable, whereby the closed-loop controls of these other internal state variables then require separate controller constants in each case.
Thus, if one wants to react to the various events in the operation of the current converter, it is customary in analog technology to set up parallel controllers and, by making an appropriate selection in each case, to only access one controller output, which then specifies the controlled variable for the delay angle of the current converter. The control loops of the controllers which are not operative in circuit are not closed thereby, so that their output signals would go up to the limit of the control modulation range, and when the controller is activated, a disproportionately long rise time would pass until the appropriate controlled variable can be optimally controlled. Therefore, the output signals from the controller must roughly match the delay angle of the current converter, in order to be able to intervene immediately in case of need.
A preferred field of application is the control of a high-voltage d.c. transmission between two stations A and B, linked in each case to a mains NA or NB, as depicted in FIG. 1.
One starts thereby from the assumption of an operating state, in which the current converter of station A works as a rectifier, and its delay angle .alpha.' specifies the firing pulses of the current converter GR by means of trigger equipment STA. In this manner, at the rectifier output, a direct voltage UdA is generated, and at a d.c. reactor, a direct current idA is generated. They are specified by suitable setpoint values (for example idA*). The control signal for the delay angle .alpha.' is therefore supplied in this operating state by a current regulator I-R', to which the system deviation idA*-idA is supplied via a linearizing element LIN'.
In station B, the current converter WR works as an inverter, which injects the dc current idB fed via a d.c. reactor and the corresponding d.c. voltage UdB via a transformer T2 into the a.c. system NB. TEL indicates a transmission line through which information is exchanged, for example concerning the setpoint value I* of the dc current exchanged between the two stations, the state of a release signal F for switching the entire system on or off, as well as concerning a signal GR/WR for the rectifier operation or the inverter operation of the two stations.
In the present operating state, in which WR is operated as an inverter, only values of .alpha.&gt;90.degree. are provided for the delay angle .alpha., which lies according to definition between 0.degree. and 180.degree.. To avoid conduction-through, the extinction angle .gamma. must be smaller than the inverter stability limit, for example 150.degree.. Since the reactive power can also be automatically controlled by means of the extinction angle, an extinction-angle controller .gamma.-R is provided, which adjusts the delay angle .gamma. in dependence upon an operationally specified setpoint value .gamma.* or the corresponding system deviation .gamma.* -.gamma..
If for reasons of temporary disturbances, the actual value idB of the injected dc current should lie below the setpoint value I* by a specified marginal value imarg below the setpoint value I*, then the extinction-angle control is supposed to be overridden by a current control. However, if the voltage UNB of the mains NB should rise above a specified limiting value UNB*, then the reactive load of the transmission length is supposed to be increased by reducing the delay angle, in other words transition is supposed to be made to a voltage control.
FIG. 1 depicts a measuring device MU for acquiring the voltage UNB, which works on a voltage regulator U-R. The corresponding measuring elements for the actual values of idB and .gamma. are not depicted, however a linearization element is shown, respectively with LIN1 and LIN2, with which a linear correlation is achieved between the system deviations .gamma.* -.gamma. of the .gamma.- controller or (I.sup.* -imarg-idB) of the corresponding current regulator I-R and the prevailing output variable of this controller.
From the output signals of the three controllers .gamma.-R, I-R and U-R, a selection element "Minimum.gamma." chooses the signal belonging to the smallest delay angle .gamma.. In this manner, the delay angle is controlled to the value which is closest to 90.degree.. The controllers themselves are operated in a way that enables them, in case of an open control loop, to supply a delay angle near the inverter stability limit. However, if the transition from the control to a U-control or an I-control is supposed to be made quickly, then the controllers, which each have an integral component, should be tracked, so that their output signals do not deviate considerably from the output signal of the .gamma. controller in operation. This is achieved in that the controllers have inputs for limiting values and the controller outputs are limited in each case to this limiting value The limiting value is stipulated thereby by a slight shift in the delay angle .alpha. by a specified differential angle .DELTA..phi..
By tracking the controller limitations in this manner, one ensures that the current regulator or the voltage regulator can quickly become operative in case of temporary disturbances.
Thus for every controlled variable in question, such a system requires a separate controller, whose controller constants are adapted to the system and whose output signal must be properly tracked. The expenditure of time and energy that this requires, increases accordingly, when still other controlled variables are supposed to be included in the specific control concept.
Generally, digital control systems can perform control tasks very precisely, however the computing time required to implement the control task between the time the actual values are sampled and the time the control signals are calculated acts as a dead time. Often, this cannot be tolerated given the computing speed of available computers.